PCB layers

The layers of PCB

The number of layers of the PCB depends on the complexity of the circuit board. From the perspective of the PCB processing process, multiple “double-sided PCBs” through stacking and laminationg, then it become a multi-layer PCB. If we distinguish  the PCB from layers we can call:

• single-sided (one copper layer),
• double-sided (one substrate layer with two copper layers on either side),
• multi-layer (outer and inner copper layers, alternating with substrate layers).

What is stack up?

The PCB designer will decide the stacking order of each PCB layer and the choice of board during the design. This order we call it “PCB stack-up design”. If the PCB has more layers, you can see more complex stack up. As picture below, it is simple 2 layers stack up. We can see from the stack up file, we will know:

1. The total PCB thickness is 1.5mm,
2. The final copper thickness is 2OZ

Which factors to consider in PCB stack-up design?

The number of layers in a PCB design and the stack-up PCB design depend on the following factors:

1. Hardware cost: The number of circut board layers is directly related to the final hardware cost. The more layers, the higher the hardware cost. Hardware PCBs represented by consumer products generally have the highest limit on the number of layers, such as notebook computer products. The number of layers of the motherboard PCB is usually 4~6 layers, rarely more than 8 layers;

2. Outgoing lines of high-density components: high-density components represented by BGA package devices. The number of outgoing layers of such components basically determines the number of wiring layers of the PCB board;

3. Signal quality control:   For PCB design with concentrated high-speed signals, if we focus is on signal quality. Likewise we need to reduce the wiring of adjacent layers to reduce crosstalk between signals.

At this time, the number of wiring layers and the number of reference layers. Ground layer or The ratio of Power layer is preferably 1:1.  Which will increase the number of PCB design layers; On the contrary, if the signal quality control is not mandatory, the adjacent wiring layer to reduce the number of the layers of PCB;

4. Schematic signal definition: The schematic signal definition will determine whether the PCB wiring is “smooth”. And a poor schematic signal definition will cause the PCB wiring to be unsmooth and the number of wiring layers to increase;

5. PCB manufacturer’s processing capability baseline:

The stacking design scheme (stacking method, stacking thickness, etc.) given by the PCB designer must fully consider the PCB manufacturer’s processing capability baseline. Such as: processing process, processing equipment capability, commonly used PCBs Plate model, etc.

PCB stack-up design needs to seek priority and balance among all the above design influencing factors.

General rules for PCB stack-up design

1. The ground layer and the signal layer should be tightly coupled. Which means that the distance between the ground layer and the power layer should be as small as possible. And the thickness of the medium should be as small as possible to increase the capacitance between the power layer and the ground layer .

If you do not understand here , you can think about the plate capacitor: the size of the capacitor is inversely proportional to the spacing.

2. Try not to be directly adjacent between the two signal layers. Which is prone to signal crosstalk and affects the performance of the circuit.

3. For multi-layer circuit boards, such as 4-layer boards and 6-layer boards. It is generally required that the signal layer be adjacent to an internal electrical layer as much as possible. So that the large-area copper cladding of the internal electrical layer can be used. The function of shielding the signal layer can effectively avoid the crosstalk between the signal layers.

4. For the high-speed signal layer, it is generally located between the two internal electrical layers. The purpose of this is to provide an effective shielding layer for the high-speed signal on the one hand.  And to limit the high-speed signal to the two internal electrical layers on the other hand. Between layers to reduce interference to other signal layers.

5. Consider the symmetry of the stacked structure.

6. Multiple grounded inner electrical layers can effectively reduce the grounding impedance.

Recommended Stacked Structure-pcb layers

1. Lay the high-frequency traces on the top layer to avoid the introduction of inductive inductance. It is due to the use of vias in the process of high-frequency traces. On the top layer, isolator and the data lines of the transmit and receive circuits are directly connected with high frequency traces.

2. A ground plane is placed under the high frequency signal line. It is to control the impedance of the transmission connection line. And, also provide a very low inductance path for the return current to flow.

3. Place the power plane under the ground plane. These two reference layers form an additional high frequency bypass capacitor of approximately 100pF/inch2.

4. Layout low-speed control signals on the bottom wiring. These signal lines have a large margin to withstand impedance discontinuities caused by vias, which allows for more flexibility.

▲Example of four-layer board stack-up design

If need additional power supply layers (Vcc) or signal layers. Moreover, we need add second set of power/ground layers to stacked symmetrically. In this way, the laminated structure is stable and the board will not warp. The power and ground planes of different voltages should be close together. So that high-frequency bypass capacitors are added to suppress noise.

Reminder: There is another layer here, which means to use even-numbered layers of PCB, avoid using odd-numbered layers. Because odd-layered boards are prone to flexing.